Although balance and gait disorders are one of the most common and devastating chronic medical problems of older people, especially those with chronic neurological diseases, clinicians or clinical researchers do not measure balance and gait frequently, accurately, or sensitively. Our long-term goal is to develop and commercialize a unique system called the Home Objective Mobility Exam (HOME) to allow patients to measure their own balance and gait at home. If physicians and physical therapists could obtain frequent, accurate, self-administered measures of balance and gait characteristics, then they will be able to earlier predict fall risk and changes in mobility. This will also enable them to quickly improve interventions in order to prevent mobility disability. The objective of this application is to deveop a novel method to allow patients to quantify their own standing balance, gait and turning with a quick, simple, and extremely easy to use system. The specifc aims are: Aim I. To develop a new version of the Opal movement monitor for home use. The objective of this aim is to transform the Opal sensors from a computer-based clinician/researcher-controlled system to a nonprofessional/patient-controlled system of movement monitoring. Aim II. To determine the feasibility, validity and reliability of having people with Parkinson's disease test their own balance and gait at home. We hypothesize that balance and gait, measured by patients in their homes, will be valid and reliable compared with measures administered by research assistants and with clinical scales and show very good test-retest reliability. We will initially develop HOME for patients with Parkinson's disease because they have more falls than any other neurological disorder, but we are confident that this tool will be useful for patients any form of mobility disability, such as multiple sclerosis, stroke, head injury, developmental disorders, multisensory deficits, frail elderly, etc. Based on results of this proposal, our Phase II application will thentest the ability of the HOME to more quickly detect changes in balance and gait following a change in medication or physical therapy, compared to usual care, in preparation for FDA approval and widespread marketing.